1. Field of the Invention
This invention relates generally to pistons for internal combustion engines, and methods for manufacturing the pistons.
2. Related Art
Pistons used in internal combustion engines, such as heavy duty diesel pistons, are exposed to extremely high temperatures during operation, especially along the crown of the piston. Therefore, to moderate the temperature, some pistons are designed with an open cooling gallery beneath the crown, and cooling oil is sprayed into the cooling gallery as the piston reciprocates along a cylinder bore of the engine. The oil flows along the inner surface of the crown and dissipates heat away from the crown. However, to control the piston temperature during operation, a high flow of oil must be maintained constantly. In addition, the oil degrades over time due to the high temperature of the internal combustion engine, and the oil must be changed periodically to maintain engine life. Furthermore, when the cooling gallery temperature exceeds 350° C., the oil tends to burn at a higher rate, referred to as oil coking, and adhere to the surface of the gallery.
Another option is to design the piston with a sealed cooling gallery containing cooling oil or another coolant to control the piston temperature. U.S. Pat. No. 9,127,619 discloses an example of a piston including a sealed cooling gallery partially filled with a liquid containing metal particles having a high thermal conductivity. The liquid carries the metal particles throughout the cooling gallery as the piston reciprocates in the internal combustion engine, and the metal particles remove heat away from the crown. The metal particles can re-distribute the heat flow, and thus reduce carbon deposits, coking, and oil degradation along the crown.
It is also desirable to reduce heat loss from the combustion chamber to the piston crown, in order to maintain a high temperature in the combustion chamber and achieve higher engine brake thermal efficiency. Thus, a thermal barrier coating can be applied to the piston crown for additional insulation. However, engine manufacturers continuously strive to develop new and improved methods to better maintain heat in the combustion chamber, reduce the operating temperature of the piston, and thus further improve engine brake thermal efficiency.